An investigation was made regarding the degree of bias and relative bias in estimating the prevalence and severity of periodontal disease that results from employing partial mouth examinations. The partial mouth exams that were considered were all based on the random half-mouth technique. This technique consists of examining sites in two quadrants, one randomly selected from the maxillary arch and one from the mandibular arch. Three large epidemiologic data sets in which full mouth periodontal measurements were collected were used in this study. True prevalence and severity of the disease was based on the full- mouth examination. This consisted of diagnosing 112 sites, based on four sites for each of the 28 teeth (excluding third molars). The sites examined were the mesiobuccal, midbuccal, distobuccal and midlingual. Four separate partial-mouth examinations were considered: the 1st evaluated mesial sites only (M), the 2nd the mesiobuccal and midbuccal sites (MB), the 3rd the mesiobuccal, midbuccal and distobuccal sites (MBD), and the 4th the mesiobuccal, midbuccal distobuccal, and midlingual sites (MBDL). All four partial-mouth examinations produced underestimates of the prevalence of periodontal disease, whereas the estimates of severity were dependent upon which sites were selected.